The present invention relates to an improved apparatus for making halogen gas, e.g. chlorine, alkali metal hydroxide and hydrogen electrolytically in a diaphragm type chlor-alkali cell equipped with a novel brine feed distribution system. The cell comprises a unitary container having a base for retaining a plurality of metal anodes, a cathode can and a cell cover. A plurality of repeating reaction zones are formed within the cell, each zone comprising an anode compartment having a dimensionally stable metal anode, a cathode compartment having a foraminous metallic cathode and a diaphragm separating both anode and cathode compartments. One or more interior brine feed distribution lines at the top or bottom of the cell has individual outlets adjacent to such reaction zones for feeding brine evenly to the anolyte compartment of each reaction zone. On the application of direct electrolyzing current to aqueous solutions of alkali metal chloride, hydrogen and alkali metal hydroxide are produced at the cathode and chlorine, substantially free of oxygen and other gas impurities, is produced at the anode.
Electrolytic cells commonly employed commercially for the conversion of alkali metal chlorides into alkali metal hydroxides and chlorine may be considered to fall into the following general categories: (1) diaphragm, (2) mercury, and (3) membrane cells.
Diaphragm cells utilize one or more separators permeable to the flow of electrolyte solution but impervious to the flow of gas bubbles. The diaphragm separates the cell into two or more compartments. Although diaphragm cells achieve relatively high product per unit floor space, at low energy requirements and at generally high current efficiency, the alkali metal hydroxide product, or cell liquor, must be concentrated and purified. Such concentration and purification is usually accomplished by a subsequent evaporation step.
Mercury cells typically utilize a moving or flowing bed of mercury as the cathode and produce an alkali metal amalgam in the mercury cathode. Halide gas is produced at the anode. The amalgam is withdrawn from the cell and treated with water to produce a high purity alkali metal hydroxide.
Membrane cells utilize one or more membranes or barriers separating the catholyte and the anolyte compartments. The membranes are permselective, that is, they are selectively permeable to either anion or cation. Generally, the permselective membranes utilized are cationically permselective. Usually, the catholyte product of the membrane cell is of a relatively high purity alkali metal hydroxide, ranging in concentration from about 250 to about 350 gpl.
Chlorine and alkali metal hydroxides are essential and large volume commodities and are recognized as basic industrial chemicals. Plants producing 500 to 1,000 tons of chlorine per day are not uncommon. Such plants typically utilize a large number of individual electrolytic cells having high current capacities. Thus, seemingly even minor improvements in individual cell operation or performance will have major economic benefits because of the volume of products produced. For example, currently metallic anodes are used almost exclusively in diaphragm cells for the electrolysis of alkali metal chloride. Compared with diaphragm cells equipped with graphite anodes, metallic anodes provide lower cell voltages, and correspondingly, lower current consumption under otherwise identical operating conditions. The lower cell voltages are achieved by narrower distances or gaps between individual anodes and cathodes.
Notwithstanding the substantial improvements made in lowering power consumption in the operation of diaphragm cells, with ever increasing energy costs further attempts are being made to reduce the current consumption by operating the cells at lower current densities, i.e. . . . at lower current intensity per surface unit. For example, it would be possible to reduce the cell voltage from about 3.4 volts to about 3.1 volts by reducing the current density from the currently standard value of about 2.3 kA/m.sup.2 to 1.5 kA/m.sup.2. This is equivalent to a theoretical electrical saving of about 10 percent. In reality, the saving in electricity is, however, much smaller mainly because of secondary reactions. Generation of anodic oxygen occurs increasingly with a decrease in current density, which results in a reduction in the amount of chlorine generated per KWH. Therefore, not only is it impossible to reach the theoretically possible power savings, but the purity of the gaseous chlorine produced in such cells also diminishes due to the increasing concentration of oxygen.
Accordingly, it has now been discovered that further improvements in electrical power savings and product purities can be achieved by feeding alkali metal chloride brine evenly into the individual anolyte compartments of an electrolytic cell. In practice, at least one brine feed conduit or brine distributor located below or above the cell's electrodes discharges brine to individual reaction zones. Sparging electrolyte directly into the reaction zones provides higher current efficiencies and higher chlorine purity.
Previous methods call for filling the cell container with electrolyte solution to above the top edge of the electrodes. Fresh brine is continuously fed onto the surface of the brine charge by means of a pipe opening in the cover of the cell or conduit in the bottom of the cell container. In each instance, however, power consumption is high and/or chlorine purity is low.
Thus, the present invention has as its principal objective improving the current efficiency and the concentration of chlorine produced in cells equipped with metallic anodes, especially during operation of cells at lower current densities.
Yet, still another object of the present invention is an improved diaphragm cell in the electrolytic production of chlorine, caustic soda and hydrogen.
These and other objects, features and advantages of this invention will become apparent to those skilled in the art after a reading of the following description.